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oxide, human-produced chlorofluorocarbons, and their substitutes. The amounts of

all these gases are increasing at a rate sufficient to have substantial climatic

consequences.

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Today, I will emphasize only the estimated climatic effects of these increasing greenhouse gases for a time around the year 2050. The year 2050 is roughly the time that current projections lead to an equivalent doubling of carbon dioxide levels. However, please note that such projections indicate that carbon dioxide amounts would continue to increase well beyond the year 2050, perhaps even to a

quadrupling.

Because I speak with credentials as a physical scientist, I do not offer personal opinions on what society should do about these predicted climate changes. Societal actions in response to greenhouse warming involve value judgements that are beyond the realm of climate science. Indeed, I would encourage your skepticism whenever you hear a climate scientist's prediction being accompanied by a policy opinion.

At the onset, please recognize that a major international effort to assess climate warming was completed in 1990. This is "The IPCC Assessment". That report involved nearly two years and 170 scientists from 25 countries. Also, it has recently been updated in the 1992 and 1994 Supplements. These Supplements reinforce the general conclusions of the 1990 Assessment. They have reduced our uncertainty in

several important areas. These are the most widely accepted statements ever on climate change. A new IPCC Assessment will be completed in the next few months. I expect only small changes in its major conclusions, mainly concerning some increases in scientific confidence.

I strongly recommend your use of these scientific assessments as a foundation for your own evaluations. I also recommend their use as a point of departure for evaluating the credibility of opinions that disagree with them.

Occasionally, my own judgements will differ somewhat from these IPCC Assessments. Overwhelmingly, however, I agree with them. I emphatically support their highest priorities for climate change research.

My information is derived from the strengths and weaknesses of climate models, climate theory, and widespread observations of the climate system. Climate models have improved in their ability to simulate the current climate and its natural variability. Unfortunately, important uncertainties remain due to deficiencies in our scientific understanding and in computer power. However, significant progress is expected over the next 10 years. The uncertainty in such areas as ocean circulation, cloud processes, and regional effects will gradually diminish.

However, let me say at the outset: None of the uncertainties I will discuss can make current concerns about greenhouse warming go away. This problem is very real and will be with us for a very long time.

I will give my evaluation of current model predictions of climate change in the middle of the next century using simple "betting odds". By "Virtually Certain", I mean that there is no plausible alternative; in effect, the bet is off the books. "Very Probable" means I estimate about a 9 out of 10 chance that this will happen within the range predicted; "Probable" implies about a 2 out of 3 chance. "Uncertain" means a plausible effect, but which lacks appropriate evidence. Essentially, I set the odds; you choose your bet. My predictions follow in decreasing levels of confidence.

Human-Caused Increasing Greenhouse Gases (virtually certain)

There is no real remaining doubt that increasing greenhouse gases are due to human activities. Clear contributors are the burning of fossil fuels and the use of chlorofluorocarbons.

Radiative Effect of Increased Greenhouse Gases (virtually certain)

Greenhouse gases absorb and reradiate infrared radiation. Independent of other factors, this property acts to produce an increased heating effect on the planet.

Large Stratospheric Cooling (virtually certain)

The combination of reduced ozone and increased carbon dioxide will lead to a

marked lowering of upper stratospheric temperatures, within the range of 15°. 30° fahrenheit.

Long Time to Draw Down Excess Carbon Dioxide (virtually certain)

We know that it takes decades to centuries to produce the buildup of greenhouse gases. Much less appreciated is that a "return to normal" from high carbon dioxide levels would require many additional centuries.

Global Surface Warming over the Past Century (virtually certain)

The observed warming in the surface temperature records of about one degree fahrenheit cannot yet be unambiguously ascribed to greenhouse warming. However, no other hypothesis is nearly as credible.

Natural Variability Adds Confusion (virtually certain)

Climate varies naturally on time scales from months to centuries. This effect makes a greenhouse warming interpretation of the climate record much more

difficult.

Global-Mean Surface Warming (Very Probable)

For the middle of the next century, global-mean surface warming is estimated to be in the range of 2 to 6° fahrenheit. The largest uncertainty is due to the effects of clouds. My own opinion is the warming is more likely to be in the lower half of this range, mainly due to cooling offsets from sulfate aerosols and ozone depletion.

Global-Mean Precipitation Increase (Very Probable)

As the climate warms, the rate of evaporation should increase, leading to an increase in global-mean precipitation. Nevertheless, some local regions would experience decreases in precipitation.

Reduction of Northern Sea Ice (Very Probable)

As the climate warms, total sea ice should decrease in high latitudes of the Northern Hemisphere. However, recent atmosphere-ocean models calculate a resistance to climate change at high Southern Hemisphere latitudes. Thus, little change in sea-ice cover may occur there over the next century.

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